Conventional laser welding works well in joining varying thicknesses of steel, but can be problematic when applied to aluminum due to the reactivity of molten aluminum to air. Current welding technologies for production of aluminum TWBs are utilized in low-volume and niche applications, and they have yet to be scaled for the high-volume vehicle market. Manufacturers today thus must create several components from single aluminum sheets that are then riveted together after being stamped, resulting in additional production steps and more parts that drive up cost and weight. The PNNL study targeted weight reduction, part reduction, and cost savings by enabling tailor-welded blank technology for aluminum alloys at high volumes.

The introduction of high-speed friction-stir welding (FSW) at linear velocities amenable to high-volume automotive production has the potential to revolutionize the current joining and assembly paradigm for aluminum stampings. Although laser welding has effectively enabled the welded blank market for steel, the unique metallurgical challenges associated with fusion/laser welding aluminum have prevented the adoption of laser welding technologies for aluminum blanks. This is evidenced by a significant increase in the use of more expensive riveting technologies in the assembly of stamped aluminum panels rather than use of welded blanks. An effective high-speed welding technology would enable increased weight reduction through the use of tailor-welded blanks (TWBs), which allow for part simplification, mass savings, and decoupled assembly at a reduced cost. Simply put, high-speed FSW of aluminum-welded blanks has potential to simultaneously reduce both mass and cost.

… For more than a decade, research related to the production of aluminum TWBs has been available. Specific studies describing the weldability of aluminum sheets have characterized numerous joining techniques including laser, electron beam, gas tungsten arc, and FSW. Each methodology has been shown to have specific advantages and disadvantages associated with the joining of aluminum alloys, which overall have proven much more problematic than welding automotive steel sheets. These difficulties are inherent in the physical chemistry of aluminum alloys and are most apparent when striving to manage a weld pool of molten aluminum. In this state, aluminum maintains a higher affinity for hydrogen than the surrounding atmosphere, so the association of hydrogen in the weld pool is common even with the use of traditional cover gases. Exfoliating during the solidification process, these gas particles often leave deleterious volumetric defects in their wake. This behavior is exacerbated by the presence of organic lubricants that are used transport aluminum sheets. These lubricants can become trapped on the welding edge of a sheet during the shearing process prior to welding, and therefore they are not eliminated with the use of cover gases during the welding process. As such, they are present during the fusion process, allowing the greater affinity of the molten aluminum to claim the available hydrogen atoms.

Additional difficulties have challenged traditional welding techniques, including increased thermal diffusivity when compared with ferrous materials, overall challenging solidification kinetics, molten viscosities, surface oxide, and so on. As such, the introduction of more novel welding techniques, such as FSW, has been commonly investigated; numerous researchers have reported successful weldability devoid of historic challenges. FSW, a solid-state joining process, avoids the difficulties associated with melting and solidification. Because welding occurs below the melting point, this process takes advantage of the reductions in both yield stress and flow stress to locally extrude the interface of the aluminum sheets into a seamless joint.

—Hovanski et al.

A friction-stir welding machine looks and acts like a cross between a drill press and a sewing machine. Lowered onto two metal sheets sitting side-by-side, the pin tool spins against both edges. As it travels along, the pin creates friction that heats, mixes and joins the alloys without melting them.

However, FSW speeds used for production (less than 1 meter/min) are generally significantly lower than travel speeds of commercial laser welding technologies, which regularly weld at speeds from 6 m/min to 10 m/min. According to the authors, the majority of data available for aluminum blanks produced via FSW were produced at welding speeds below 0.5 m/min. Although other research has explord speeds beyond 1 m/min, these only cover limited thickness ratios up to 1.5:1.

In the study, the team joined dissimilar thickness AA5182-O sheets of 1.2 mm and 2.0 mm thicknesses, using different tool geometries. The only constant parameter used in the development of the high-speed FSW was a fixed linear velocity (welding speed) of 3 m/min. All weld process development was performed on a high-precision FSW machine located at PNNL.

Prototypical door inner panel stamped from AA5182-O TWB including (a) a stack of as-welded TWBs, (b) laser-trimmed TWB, (c) TWB positioned in the stamping die, and (d) stamped differential gauge aluminum door inner panel. Hovanski et al. Click to enlarge.

In all, dozens of unique tool designs with varying shapes, lengths and diameters of the pin were created. These were assessed against a variety of weld parameters, such as the depth, rotation speed and angle of the tool. Through statistical analysis, the team identified the optimal combination of tool specification and weld parameters that could consistently withstand high-speed production demands.

PNNL provided the weld and tool specifications to TWB Company and GM. TWB Company then independently welded, formed and analyzed more than 100 aluminum blanks in close coordination with GM, making them the first qualified supplier of aluminum tailor-welded blanks. GM subsequently stamped their first full-sized inner door panel supplied by TWB Company—free of imperfections—from aluminum sheets in varying thicknesses. The resulting door is 62% lighter and 25% cheaper than that produced with today’s manufacturing methods.

Today, TWB Company has a dedicated FSW machine at their production facility in Monroe, MI, built around PNNL’s process that is capable of producing up to 250,000 parts per year.

TWB can now provide aluminum tailor welds not only to GM, but the entire automotive industry.

—Blair Carlson, a group manager at GM who con-conceptualized the project

With over two years of funding left, the team continues to collaborate, with a focus on even faster weld speeds and the ability to maneuver around the contours and corners of complex aluminum parts, for which laser welding is not commercially feasible. The team also is modifying FSW to join different alloys, such as automotive-grade aluminum alloys with light, ultra-high strength alloys currently reserved for aerospace applications.

Going forward, we see this process, and future versions of it, enabling completely novel combinations of materials that will revolutionize material use in the auto industry.

—Yuri Hovanski, PNNL program manager and lead author

The two-phase, six-year project is funded by the Department of Energy’s Office of Energy Efficiency and Renewable Energy with in-kind partner contributions from each of the participating companies.

Resources

• Y. Hovanski, P. Upadhyay, J. Carsley, T. Luzanski, B. Carlson, M. Eisenmenger, A. Soulami, D. Marshall, B. Landino, S. Hartfield-Wunsch. High-speed friction-stir welding to enable aluminum tailor-welded blanks, JOM, doi: 10.1007/s11837-015-1384-x.